Introduction / Background / Justification: Plant growth rates drive landscape productivity and carbon sequestration, and thus understanding what drives them is essential in determining how vegetation will respond to human-induced disturbances and climate change. Objective(s)/Hypothesis(es): Here we test predictions for how commonly studied leaf and wood tissue traits, as well as nutrient concentration in leaves and wood, affect tree stem diameter growth rates in two major tropical forest formations of South America, the Amazon and Atlantic Forest. We also tested how this effect varies across undisturbed primary and logged secondary forests, and to verify how the extreme drought event in 2015/2016 affected this relationship across the most representative species of plant communities. Methods: All plots are part of the GEM intensive carbon plot network and were monitored regularly since 2010 following standard RAINFOR protocol. Leaf and wood traits were collected from species representing the 80% basal area in each site. Growth rates were calculated using the diameter increment inventories, considering the period before (2010-2015) and after drought (2016-2019). We used generalized mixed effect models to test for the effect of different disturbances (undisturbed and secondary forests) and the distinct functional traits in growth rate before and after the drought. Results: Despite tree size, which was a major predictor in all sites, we found that both Amazon and Atlantic Forest tree growth rates were more associated with phosphorus concentration. Species from secondary forest had 2-fold higher average growth rates than species from undisturbed sites and were more affected by the extreme drought. In the two biomes, species from secondary forests showed lower growth rates after the drought (~ 60% and ~35% lower in Amazon and Atlantic Forest, respectively), while trees in undisturbed sites maintained similar growth rates compared to previous years. Undisturbed forest tree growth was mostly associated with specific leaf area, while secondary forest tree growth was better predicted by tree height. After the drought, higher growth rates were mainly driven by leaf and wood P concentration in both undisturbed and secondary forests. Implications/Conclusions: Despite small differences between the two biomes, we found noticeable differences between undisturbed and secondary forests, suggesting that disturbance alter species growth rate and their growth-trait relationship. Moreover, disturbance modify forest resilience to extreme drought events, by directly affecting growth rates and converging the main drivers of tree growth, where phosphorus use and storage may play a leading role in tropical forest recovery.
Growth, Drought, Amazon, Atlantic Forest, Nutrients, Wood density